An optical navigation device, such as an optical mouse, typically includes a light source to illuminate a navigation surface and an image sensor that functions as a miniature digital camera to continually collect images of the illuminated navigation surface and to determine the speed and direction that the device is being moved across the surface by comparing sequentially recorded frames of image information. Image frames are collected at a very high rate and the resolution of the image sensor is high enough to detect very small movements of the device relative to the navigation surface.
When an optical mouse is used on most opaque surfaces, such as desktops, the collected image frames have enough features to determine relative movement between image frames. However, using an optical mouse on a transparent surface, such as a plate of glass that sits on a desktop, presents unique challenges. In particular, the top surface of the glass plate is typically so smooth that it provides few if any distinguishable features in the collected image frames and the thickness of the glass can change the geometry between the light source, the image sensor, and the underlying desktop such that an insufficient amount of light is reflected from the desktop to the image sensor.
An optical mouse could be designed for dedicated use on a desktop that is covered by a glass plate of known thickness. Although such an application-specific design is possible, it is not practical because an optical mouse may be used on both transparent and opaque surfaces over its lifetime and because the thickness of glass plates that cover desktops is difficult to predict and may change from plate to plate.